A DC converter is a device that converts an unregulated source of DC electrical energy into a source of constant DC voltage or current. The converter typically includes a transformer, having primary and secondary windings wound around a common magnetic core. The current or voltage applied to the primary winding is increased or decreased by the transformer in proportion to the ratio between the number of turns included in the primary and secondary windings. Regulation of the output voltage is achieved with the aid of a controlled switch or switches connected in the primary circuit. More particularly, by opening and closing the primary circuit for appropriate intervals, precise control over the energy transfer between primary and secondary is accomplished. For example, where an increase in the voltage or current at the output is required, the interval during which the primary is conducting can be increased. On the other hand, relatively long interruptions in the flow of current through the primary winding result in lower voltages or currents at the output.
Although a number of different DC converter constructions or topologies exist, the basic arrangement of interest, shown in FIG. 1 without a core discharging circuit, is commonly known as a single-ended, forward, DC converter. The single-ended designation indicates that power flow in the primary winding of the converter transformer is gated by one active device. In the traditional single-ended construction shown in FIG. 1, such a converter exercises the transformer over only one-half of its magnetization or B-H curve, where B and H equal the flux density and magnetic field intensity, respectively, in the core.
In comparison, a push-pull arrangement employs two active devices to conduct current through the primary winding during alternative half cycles. The two switches operate in opposite phase with respect to each other and the output of the arrangement is regulated using duty control. Such an arrangement exercises the transformer core over its entire magnetization curve, producing flux in the core having both positive and negative values. Disadvantages of this construction, however, are that the switches may cross-conduct and the core may "walk" into saturation.
The forward designation indicates that the primary and secondary windings of the transformer are simultaneously connected to the voltage source and load, respectively. As a result, when the primary winding is closed, energy is transferred "forward" through the transformer, from primary to secondary.
Addressing now the construction and operation of a basic single-ended, forward, DC converter, as shown in FIG. 1 it includes a transformer T having a primary winding P that is magnetically coupled to a secondary winding S by a magnetic core M. An input voltage source V.sub.in is applied to the series combination of the transformer primary P and a controlled, main switch SW1. An AC rectifier and filter circuit, including rectifier diodes RD1 and RD2, filter inductor FL, and filter capacitor FC, is coupled across the secondary winding S. As suggested previously, by opening and closing switch SW1 for appropriate intervals, the unregulated source V.sub.in can be converted to the desired regulated output V.sub.o.
As will be appreciated, a "magnetizing" component of the current in the primary winding P furnishes the magnetomotive force required to overcome the magnetic reluctance of the core M. This magnetizing current causes energy to be stored in the transformer core M when the main switch SW1 is closed. When switch SW1 opens, the core must be reset by discharging the stored energy.
Numerous ways of discharging the core energy in single-ended, forward, DC converters have been developed. Traditionally, a demagnetizing winding DM has been employed as shown in FIG. 2. The demagnetizing winding DM has a polarity that is inverted with respect to the primary and secondary windings P and S of transformer T and is connected in series with a blocking diode D across the series combination of primary winding P and main switch SW1. Operation of demagnetizing winding DM is as follows.
With the switch SW1 closed, the source voltage V.sub.in is applied across the primary winding P of transformer T. During this interval, energy is stored in the transformer core M as a result of the magnetizing current flowing in the primary winding P. Because diode D is reverse biased, it blocks the flow of current through the demagnetizing winding DM. Once switch SW1 is opened, the current in the primary winding P is interrupted. At this time, the diode D becomes forward biased and the transformer core M is reset, or discharged, as the magnetic energy stored in the core M induces a current in the demagnetizing winding DM.
Assume a one-to-one turn ratio between the primary and demagnetizing windings P and DM. The current in the demagnetizing winding DM is initially equal to the peak value of the magnetizing current and decreases linearly over time until the energy stored in the core has been returned to the voltage source. The reset interval is equal to the period during which the switch SW1 was closed. After the core M has been discharged, but prior to the reclosure of switch SW1, the current through the demagnetizing winding DM is equal to zero because the energy stored in the core has been discharged and because no current is induced by the voltage V.sub.in applied across the series combination of the demagnetizing winding DM and diode D.
While the circuit of FIG. 2 has improved converter efficiency by transferring energy from core M back to the source V.sub.in when the switch1 is open, it presents several problems. First, the use of a demagnetizing winding DM to discharge core M involves an inherent duty cycle limitation. More particularly, assuming a one-to-one ratio between the number of turns in the primary and demagnetizing windings P and DM, the period of time required to "charge" the core M is equal to the time required to "discharge" the core M. As a result, if all the energy is to be discharged from the transformer core M and converter failure avoided, the switch SW1 must be open during each cycle for at least as long as it is closed. This limits the maximum duty cycle achievable with a one-to-one turn ratio to 50%. While an increased duty cycle can be achieved by altering the turns ratio between the primary and demagnetizing windings P and DM, this approach has the disadvantage of producing higher voltage peaks across the switch SW1. As will be appreciated, a widely variable duty cycle is desirable because it allows the converter to regulate the voltage applied to a broad range of loads from a widely varying source.
An alternative method of resetting the core, which overcomes the duty cycle and voltage peak limitations of the demagnetizing winding arrangement shown in FIG. 2 is described in U.S. Pat. No. 4,441,146. As shown in FIG. 3, the particular circuit of interest disclosed by this reference is an adaptation of the single-ended, forward, DC converter illustrated in FIG. 1. More particularly, the series combination of a capacitor C and switch SW2 is placed in parallel with the primary winding P of transformer T. The operation of switch SW2 is controlled by a control circuit CC such that, when the main switch SW1 is open, the capacitor C is coupled to winding P to form a resonant circuit with the transformer's magnetizing inductance. This resonant circuit resets the magnetizing energy stored in the core M by creating a "mirror image" of the magnetic flux between the periods during which switch SW1 is closed. As a result, capacitor C, switch SW2 and control circuit CC are collectively referred to as a "magnetizing current mirror."
Addressing the operation of this magnetizing current mirror in slightly greater detail, reference is had to FIGS. 4, 5, and 6, in which the operation of switch SW1, the voltage V.sub.s across switch SW1 and the current I.sub.c through capacitor C, respectively, are illustrated as a function of time. The capacitance of capacitor C is sufficiently large to render the time dependence of the voltage V.sub.c across capacitor C negligible when switch SW2 is closed. As a result, the voltage V.sub.s across the open switch SW1 will be constant and the current I.sub.c flowing through capacitor C will rise linearly, as described below.
The operation depicted in FIGS. 4, 5, and 6 is for a 33% duty cycle in which primary switch SW1 is closed between times t.sub.l and t.sub.2 and open between times t.sub.2 and t.sub.4. The control circuit CC operates in conjunction with switch SW1 to ensure that switch SW2 opens prior to the closing of switch S1 and closes after switch SW1 opens. As shown in FIG. 5, when switch SW1 is closed, the voltage V.sub.s across switch SW1 and the current I.sub.C through the second switch SW2 are both equal to zero. During this interval, the source voltage V.sub.in is applied to the primary winding P of transformer T, inducing a current flow in the secondary winding S and storing energy in the transformer core M. Once switch SW1 is opened, no current flows through it, and the associated closing of switch SW2 will cause the voltage across that switch to drop to zero. The voltage V.sub.s across switch SW1 is clamped to a value V.sub.p that is equal to the sum of the input voltage V.sub.in and the voltage V.sub.c across capacitor C1.
When switch SW1 is opened, magnetizing energy stored in the core M while switch SW1 was closed is transferred to capacitor C. Initially, the current I.sub.c flowing through capacitor C is negative in sign and equal to the peak magnetizing current I.sub.p. During the interval defined by times t.sub.2 and t.sub.3, magnetizing current is transferred from the transformer T to capacitor C, charging the capacitor C. At time t.sub.3, this magnetizing current vanishes. Then, between times t.sub.3 and t.sub.4, the stored magnetizing energy is transferred back from capacitor C to the transformer T. The process is complete at time t.sub.4, when the magnetizing energy has been reflected back into the transformer T, resetting it. A wide range of duty cycles can be achieved by selecting the appropriate source voltage V.sub.in and capacitor C.
It is noted in U.S. Pat. No. 4,441,146 that the magnetizing current mirror described above not only advantageously recycles the core's magnetization energy, making use of the available flux swing, while minimizing voltage stress on the switch SW1 during the off period by avoiding dead time, but also eliminates constraints on the converter duty cycle. It is stated that the current mirror can be connected in parallel with either the primary or secondary winding P or S of the transformer T.
While the circuit of FIG. 3 does accomplish the desired resetting of core M without the disadvantages attributable to the use of the demagnetizing winding arrangement illustrated in FIG. 2, it is not without problems. For example, in a transistor implementation of switch SW2, the control signal applied to switch SW2 must be referenced to either the positive terminal of the unregulated source V.sub.in or the capacitor C. In addition, the application of separate control signals to the two switches SW1 and SW2 makes it difficult to ensure that both switches are not on simultaneously as is required for proper operation. Further, a fault in the control circuit CC could lead to the simultaneous closure of switches SW1 and SW2, preventing proper core discharge and leading to failure of the converter.
In light of the foregoing discussion, it would be desirable to produce a single-ended, forward, DC converter constructed to discharge transformer core energy without affecting control over the duty cycle of the converter, while simultaneously providing simple, effective control of the converter switching elements.